With rapid development in image display technology, touch screens that allow data input using direct contact have become common display apparatuses and widely applied to various electronic products.
Nonetheless, most conventional touch screens can only detect a single touch point at a time. Once a user indicates two or more touch points on the touch screen simultaneously, a detection error will occur. Additionally, such conventional touch screens usually have small touch sensing area.
As interactive whiteboard application is getting more popular, there exits a strong need in providing an interactive input system capable of handling multiple inputs and having large touch sensing area. Currently, optical touch detection technology is considered as an effective means for achieving the abovementioned needs.
The optical touch detection technology is based on detection of light traveling in optical paths on or above the touched surface, and uses combinations of optical sensors, optical emitters, digital signal processing, and algorithms to determine a touch point. Generally, two optical assemblies are positioned along edges of a touch sensing area, with their fields of view covering the touch sensing area. The optical sensors are oriented to track any object movement within the touch sensing area by detecting interruptions of light within their fields of view. In most cases, both the optical sensor and optical emitter, such as light emitting diode, are incorporated within one optical assembly.
Some optical touch detection systems further include retro-reflective materials positioned around the touch area for reflecting or guiding light from the optical emitter back toward the optical sensors. It is well known in the art that the retro-reflective materials, mostly mounted on bezel segments, are able to return light in substantially the same direction from where the light is originated.
Once an object interrupts light in the detection plane, the object will cast a shadow on the bezel segment, which is registered as a decrease in retro-reflected light. With this principle, a first optical sensor would register the location of shadow to determine the direction of the first shadow cast on bezel segment. Meanwhile, a second optical sensor would register the location of second shadow cast on the bezel segment within its field of view.
Based on the light interruption, a touch point can be triangulated from the intersection of the two shadow lines. With further calculation, a coordinate of the touch point is determined.
Nevertheless, a problem arises when two or more points are simultaneously touched. For example, two touch points result in four shadows at the edges of touch area, leading to four intersections. Two of them are real touch points, while the other two are considered as the ghost points. With respect to the conventional triangulation algorithms used for calculating touch point coordinates, the ghost points and touch points appear as potential points, which have to be further evaluated to determine which of them are the true touch points.
A conventional way to distinguish between ghost points and true touch points is by increasing the number of optical sensors positioned along the touch area. For example, four optical sensors are used and positioned at four corners of the touch area. Even increasing the number of optical sensors can improve triangulation accuracy, it also increases the number of potential points, which have to be evaluated to obtain the true touch points. For example, when there are two true touch points and four optical sensors, 24 potential points are generated. With four touch points, 96 potential points are generated.
A potential point pair is a pair of two potential points. If the respective potential points within a pair are close together, it is likely that the pair represents a true touch point. Thus, in order to find true touch points, the analysis requires searching all combinations of potential point pairs that are the least apart and selecting true touch points from this set by binning and sorting by frequency.
Assuming with four potential points, there are 6 combinations of potential point pairs that have to be computed for the parting distance between the potential points within each pair. For 96 potential points, there will be 4,560 potential point pairs generated. As described above, once all potential point pairs are identified, the distance between each potential points within each pair is computed, then the computed distances are compared and sorted in order to determine which pairs representing the true touch points. Such analysis is computationally intensive, especially when dealing with many touch points simultaneous.
Apart from the difficulty in elimination of ghost points, once all sensing modules are exposed simultaneously, over-exposed signal and interference effect may appear since light emitted from a sensing module can interfere the signals received by the others.
US2011/0205189A1 discloses a method and system for resolving multi-touch scenarios based on calculating the distance between two potential points obtained from two pairs of optical sensors. However, such computation is inefficient, overly resource and time consuming.
As disclosed in US2011/0169727A1, sensing modules are exposed sequentially by reducing the light intensity emitted by certain sensing module for avoiding generation of potential points and interference effect during signal detection. Nonetheless, if a touch object is moving very fast, the touch object will be captured at different positions, thereby making finding the accurate position of the touch object difficult.